I have used both the mamiya 6 and the 7 (and the MkII 7) and of them I would say the Mamiya 6 is the better camera. It is lighter, handier and has absolutely stunning lenses. There is also no need to turn the camera on its side for portarit shooting, because you shoot square, just the same as Blad and Broni 6x6 (or Rollei or anyone else come to that!)
Instead of shooting tight to the frame edges, you soon learn to see in the square format. If you are shooting for the page, then you just leave 1/3 room...there is so much quality available from the negs that cropping to suit portrait or landscape doesn't matter - even for very big prints.
The 7 was brought in at the request of stock and travel shooters who wanted a full page from their trannies with no waste, i.e. no crop required (6x7 exactly matches printed page dimensions ratio). The downside was a bigger body to accommodate the larger image circle, bigger lenses to create that larger image circle and fewer shots on the roll of film (6x6 = 12 on a roll of 120 while 6x7 only gives 10). Being a rectangular format, you then need to turn the camera to create portrait shots....and it is a lump to turn round.
The timing was at just the wrong time too, because digital started to be accepted by the major libraries soon after the 7 was released, and digital backs became available, and now most libraries don't want film stock at all, if you shoot film they want you to scan it yourslef, so from an economic standpoint, it is a loser - the time involved makes shooting stock on film expensive, in materials and time.
If you want a MF rangefinder - go for the Mamiya 6. It is a brilliant performer and you will soon get used to the rangefinder and what it sees - it is only close to shots that you need to be careful with.The RZ was a brilliant piece of machinery, but so heavy and cumbersome I was glad to change when it got nicked. I then went to the Contax 645 system, because of the functions and beautifully crisp lenses (better results, despite the smaller format, than the 6x7 from the RZ!!).....now, I haven't shot a roll of film for 4 years, other than panoramics in the X-Pan. There still isn't a handy panoramic digital camera yet.